Multi-beam inspection apparatus

ABSTRACT

A multi-beam inspection apparatus including an improved source conversion unit is disclosed. The improved source conversion unit may comprise a micro-structure deflector array including a plurality of multipole structures. The micro-deflector deflector array may comprise a first multipole structure having a first radial shift from a central axis of the array and a second multipole structure having a second radial shift from the central axis of the array. The first radial shift is larger than the second radial shift, and the first multipole structure comprises a greater number of pole electrodes than the second multipole structure to reduce deflection aberrations when the plurality of multipole structures deflects a plurality of charged particle beams.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. application 62/787,157 whichwas filed on Dec. 31, 2018, and which is incorporated herein in itsentirety by reference.

FIELD

The embodiments provided herein generally relate to a multi-beaminspection apparatus, and more particularly, a multi-beam inspectionapparatus including an improved source conversion unit.

BACKGROUND

When manufacturing semiconductor integrated circuit (IC) chips, patterndefects or uninvited particles (residuals) inevitably appear on a waferor a mask during fabrication processes, thereby reducing the yield. Forexample, uninvited particles may be troublesome for patterns withsmaller critical feature dimensions, which have been adopted to meet theincreasingly more advanced performance requirements of IC chips.

Pattern inspection tools with a charged particle beam have been used todetect the defects or uninvited particles. These tools typically employa scanning electron microscope (SEM). In a SEM, a beam of primaryelectrons having a relatively high energy is decelerated to land on asample at a relatively low landing energy and is focused to form a probespot thereon. Due to this focused probe spot of primary electrons,secondary electrons will be generated from the surface. The secondaryelectrons may comprise backscattered electrons, secondary electrons, orAuger electrons, resulting from the interactions of the primaryelectrons with the sample. By scanning the probe spot over the samplesurface and collecting the secondary electrons, pattern inspection toolsmay obtain an image of the sample surface.

SUMMARY

The embodiments provided herein disclose a particle beam inspectionapparatus, and more particularly, an inspection apparatus using aplurality of charged particle beams.

In some embodiments, a micro-structure deflector array in the inspectionapparatus includes a plurality of multipole structures, each multipolestructure comprising a plurality of pole electrodes. The micro-deflectorarray includes a first multipole structure of the plurality of multipolestructures, which has a first radial shift from a central axis of thearray, and a second multipole structure of the plurality of multipolestructures, which has a second radial shift from the central axis of thearray. The first radial shift is larger than the second radial shift.Furthermore, the first multipole structure comprises a greater number ofpole electrodes than the second multipole structure to reduce deflectionaberrations when the plurality of multipole structures deflects aplurality of charged particle beams.

In some embodiments, the micro-structure deflector array may include oneor more layers of multipole structures. A first layer of the pluralityof multipole structures comprises a first multipole structure having afirst radial shift from a central axis of the array and a secondmultipole structure having a second radial shift from the central axisof the array. The first radial shift is larger than the second radialshift. Furthermore, the first multipole structure comprises a greaternumber of pole electrodes than the second multipole structure to reducedeflection aberrations of the corresponding charge particle beams. Themicro-structure deflector array also includes a second layer ofmultipole structures of the plurality of multipole structures, whichcomprises a third multipole structure having a third radial shift fromthe central axis of the array.

In some embodiments, a method of manufacturing the micro-structuredeflector array is provided. The micro-structure deflector arrayincludes a plurality of multipole structures and each multipolestructure comprising a plurality of pole electrodes. The methodcomprises forming the first multipole structure to have a first radialshift from a central axis of the array. The method further comprisesforming the second multipole structure to have a second radial shiftfrom the central axis of the array, wherein the first radial shift islarger than the second radial shift and the first multipole structurehas a different number of pole electrodes from the second multipolestructure.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF FIGURES

The above and other aspects of the present disclosure will become moreapparent from the description of exemplary embodiments, taken inconjunction with the accompanying drawings.

FIG. 1 is a schematic diagram illustrating an exemplary charged particlebeam inspection system, consistent with embodiments of the presentdisclosure.

FIG. 2 is a schematic diagram illustrating an exemplary multi-beamapparatus that is part of the exemplary charged particle beam inspectionsystem of FIG. 1, consistent with embodiments of the present disclosure.

FIG. 3A is a schematic diagram of exemplary multi-beam apparatusillustrating an exemplary configuration of source conversion unit of theexemplary charged particle beam inspection system of FIG. 1, consistentwith embodiments of the present disclosure.

FIG. 3B is a schematic diagram of exemplary multipole structure arraywith a 3×3 configuration that is part of exemplary source conversionunit of FIG. 3A.

FIG. 4 is an illustration of distributions of radial and tangentialelectrostatic fields within a multipole structure.

FIGS. 5A, 5B, 5C, 5D, and 5E are schematic diagrams of exemplarymultipole structures.

FIG. 6A is a schematic diagram of exemplary multipole structure array,consistent with embodiments of the present disclosure.

FIG. 6B schematically illustrates grouping of multipole structures ofthe exemplary multipole structure array of FIG. 6A, consistent withembodiments of the present disclosure.

FIG. 6C schematically illustrates subgrouping of a group shown in FIG.6B, consistent with embodiments of the present disclosure.

FIG. 7A is a schematic diagram of exemplary multipole structure arraywith multiple layers, consistent with embodiments of the presentdisclosure.

FIG. 7B is a schematic diagram of an exemplary layer of multipolestructure array of FIG. 7A, consistent with embodiments of the presentdisclosure.

FIGS. 8A and 8B are schematic diagrams of an exemplary multipolestructure array with multiple layers, consistent with embodiments of thepresent disclosure.

FIGS. 8C, 8D and 8E are schematic diagrams of exemplary layers ofmultipole structure array of FIG. 8A, consistent with embodiments of thepresent disclosure.

FIG. 9 is a flow chart illustrating an exemplary method of manufacturingan exemplary configuration of a multipole structure array, consistentwith embodiments of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary embodiments, examplesof which are illustrated in the accompanying drawings. The followingdescription refers to the accompanying drawings in which the samenumbers in different drawings represent the same or similar elementsunless otherwise represented. The implementations set forth in thefollowing description of exemplary embodiments do not represent allimplementations consistent with the invention. Instead, they are merelyexamples of apparatuses and methods consistent with aspects related tothe invention as recited in the appended claims.

The enhanced computing power of electronic devices, while reducing thephysical size of the devices, can be accomplished by significantlyincreasing the packing density of circuit components such astransistors, capacitors, diodes, etc. on an IC chip. For example, an ICchip of a smart phone, which is the size of a thumbnail, may includeover 2 billion transistors, the size of each transistor being less than1/1000th of a human hair. Thus, it is not surprising that semiconductorIC manufacturing is a complex and time-consuming process, with hundredsof individual steps. Errors in even one step have the potential todramatically affect the functioning of the final product. Even one“killer defect” can cause device failure. The goal of the manufacturingprocess is to improve the overall yield of the process. For example, fora 50-step process to get to a 75% yield, each individual step must havea yield greater than 99.4%, and if the individual step yield is 95%, theoverall process yield drops to 7%.

While high process yield is desirable in an IC chip manufacturingfacility, maintaining a high wafer throughput, defined as the number ofwafers processed per hour, is also essential. High process yield andhigh wafer throughput can be impacted by the presence of defects,especially if operator intervention is required for reviewing thedefects. Thus, high throughput detection and identification of micro andnano-sized defects by inspection tools (such as a SEM) is essential formaintaining high yield and low cost.

A SEM scans the surface of a sample with a focused beam of primaryelectrons. The primary electrons interact with the sample and generatesecondary electrons. By scanning the sample with the focused beam andcapturing the secondary electrons with a detector, the SEM creates animage of the scanned area of the sample. For high throughput inspection,some of the inspection systems use multiple focused beams of primaryelectrons. As the multiple focused beams can scan different parts of awafer at the same time, multi-beam inspection system can inspect a waferat a much higher speed than a single-beam inspection system.

In a conventional multi-beam inspection system, however; increasing thenumber of focused beams means that more off-axis (not on a primaryoptical axis of the system) focused beams are employed. An off-axisfocused beam has aberrations that increase with its radial shift fromthe primary optical axis, and therefore degrades the quality of imagesthat are produced for inspection. This aberration increase is, in somecases, a consequence of the directions of some of the electron beamsneeding to be changed substantially to scan the surface of the wafer.When the number of electron beams are increased, some of the electronbeams need to be routed away from the central axis of the scanningdevice. To ensure all electron beams arrives at the surface of the waferat the right angle, these off-center electron beams are manipulated morethan the other electron beams around the central axis. This higher levelof manipulation may cause blurry and out-of-focus images of the samplewafer. One aspect of the present disclosure relates to a system and amethod of reducing aberrations of off-axis focused beams to minimizedegradation of image quality. This can be achieved by using inherentlysmall-aberration source-conversion unit.

Relative dimensions of components in drawings may be exaggerated forclarity. Within the following description of drawings the same or likereference numbers refer to the same or like components or entities, andonly the differences with respect to the individual embodiments aredescribed. As used herein, unless specifically stated otherwise, theterm “or” encompasses all possible combinations, except whereinfeasible. For example, if it is stated that a database can include Aor B, then, unless specifically stated otherwise or infeasible, thedatabase can include A, or B, or A and B. As a second example, if it isstated that a database can include A, B, or C, then, unless specificallystated otherwise or infeasible, the database can include A, or B, or C,or A and B, or A and C, or B and C, or A and B and C.

Reference is now made to FIG. 1, which is a schematic diagramillustrating an exemplary charged particle beam inspection system 100,consistent with embodiments of the present disclosure. As shown in FIG.1, charged particle beam inspection system 100 includes a main chamber10, a load lock chamber 20, an electron beam tool 40, and an equipmentfront end module (EFEM) 30. Electron beam tool 40 is located within mainchamber 10. While the description and drawings are directed to anelectron beam, it is appreciated that the embodiments are not used tolimit the present disclosure to specific charged particles.

EFEM 30 includes a first loading port 30 a and a second loading port 30b. EFEM 30 may include additional loading port(s). First loading port 30a and second loading port 30 b may, for example, receive wafer frontopening unified pods (FOUPs) that contain wafers (e.g., semiconductorwafers or wafers made of other material(s)) or samples to be inspected(wafers and samples are collectively referred to as “wafers” hereafter).One or more robot arms (not shown) in EFEM 30 transport the wafers toload lock chamber 20.

Load lock chamber 20 may be connected to a load lock vacuum pump system(not shown), which removes gas molecules in load lock chamber 20 toreach a first pressure below the atmospheric pressure. After reachingthe first pressure, one or more robot arms (not shown) transport thewafer from load lock chamber 20 to main chamber 10. Main chamber 10 isconnected to a main chamber vacuum pump system (not shown), whichremoves gas molecules in main chamber 10 to reach a second pressurebelow the first pressure. After reaching the second pressure, the waferis subject to inspection by electron beam tool 40. In some embodiments,electron beam tool 40 may comprise a single-beam inspection tool. Inother embodiments, electron beam tool 40 may comprise a multi-beaminspection tool.

A controller 50 is electronically connected to electron beam tool 40.Controller 50 may be a computer configured to execute various controlsof charged particle beam inspection system 100. Controller 50 may alsoinclude a processing circuitry configured to execute various signal andimage processing functions. While controller 50 is shown in FIG. 1 asbeing outside of the structure that includes main chamber 10, load lockchamber 20, and EFEM 30, it is appreciated that controller 50 may bepart of the structure. While the present disclosure provides examples ofmain chamber 10 housing an electron beam inspection tool, it should benoted that aspects of the disclosure in their broadest sense are notlimited to a chamber housing an electron beam inspection tool. Rather,it is appreciated that the foregoing principles may also be applied toother tools that operate under the second pressure.

Reference is now made to FIG. 2, which is a schematic diagramillustrating an exemplary electron beam tool 40 including a multi-beaminspection tool that is part of the exemplary charged particle beaminspection system 100 of FIG. 1, consistent with embodiments of thepresent disclosure. Multi-beam electron beam tool 40 (also referred toherein as apparatus 40) comprises an electron source 201, a gun apertureplate 271, a condenser lens 210, a source conversion unit 220, a primaryprojection system 230, a motorized stage 209, and a sample holder 207supported by motorized stage 209 to hold a sample 208 (e.g., a wafer ora photomask) to be inspected. Multi-beam electron beam tool 40 mayfurther comprise a secondary projection system 250 and an electrondetection device 240. Primary projection system 230 may comprise anobjective lens 231. Electron detection device 240 may comprise aplurality of detection elements 241, 242, and 243. A beam separator 233and a deflection scanning unit 232 may be positioned inside primaryprojection system 230.

Electron source 201, gun aperture plate 271, condenser lens 210, sourceconversion unit 220, beam separator 233, deflection scanning unit 232,and primary projection system 230 may be aligned with a primary opticalaxis 204 of apparatus 40. Secondary projection system 250 and electrondetection device 240 may be aligned with a secondary optical axis 251 ofapparatus 40.

Electron source 201 may comprise a cathode (not shown) and an extractoror anode (not shown), in which, during operation, electron source 201 isconfigured to emit primary electrons from the cathode and the primaryelectrons are extracted or accelerated by the extractor and/or the anodeto form a primary electron beam 202 that form a primary beam crossover(virtual or real) 203. Primary electron beam 202 may be visualized asbeing emitted from primary beam crossover 203.

Source conversion unit 220 may comprise an image-forming element array(e.g., image-forming element array 322 of FIG. 3A), an aberrationcompensator array (e.g., aberration compensator array 324 of FIG. 3A), abeam-limit aperture array (e.g., beam-limit aperture array 321 of FIG.3A), and a pre-bending micro-deflector array (e.g., pre-bendingmicro-deflector array 323 of FIG. 3A). In some embodiments, thepre-bending micro-deflector array deflects a plurality of primarybeamlets 211, 212, 213 of primary electron beam 202 to normally enterthe beam-limit aperture array, the image-forming element array, and anaberration compensator array. In some embodiment, condenser lens 210 isdesigned to focus primary electron beam 202 to become a parallel beamand be normally incident onto source conversion unit 220. Theimage-forming element array may comprise a plurality of micro-deflectorsor micro-lenses to influence the plurality of primary beamlets 211, 212,213 of primary electron beam 202 and to form a plurality of parallelimages (virtual or real) of primary beam crossover 203, one for each ofthe primary beamlets 211, 212, and 213. In some embodiments, theaberration compensator array may comprise a field curvature compensatorarray (not shown) and an astigmatism compensator array (not shown). Thefield curvature compensator array may comprise a plurality ofmicro-lenses to compensate field curvature aberrations of the primarybeamlets 211, 212, and 213. The astigmatism compensator array maycomprise a plurality of micro-stigmators to compensate astigmatismaberrations of the primary beamlets 211, 212, and 213. The beam-limitaperture array may be configured to limit diameters of individualprimary beamlets 211, 212, and 213. FIG. 2 shows three primary beamlets211, 212, and 213 as an example, and it is appreciated that sourceconversion unit 220 may be configured to form any number of primarybeamlets. Controller 50 may be connected to various parts of chargedparticle beam inspection system 100 of FIG. 1, such as source conversionunit 220, electron detection device 240, primary projection system 230,or motorized stage 209. In some embodiments, as explained in furtherdetails below, controller 50 may perform various image and signalprocessing functions. Controller 50 may also generate various controlsignals to govern operations of the charged particle beam inspectionsystem.

Condenser lens 210 is configured to focus primary electron beam 202.Condenser lens 210 may further be configured to adjust electric currentsof primary beamlets 211, 212, and 213 downstream of source conversionunit 220 by varying the focusing power of condenser lens 210.Alternatively, the electric currents may be changed by altering theradial sizes of beam-limit apertures within the beam-limit aperturearray corresponding to the individual primary beamlets. The electriccurrents may be changed by both altering the radial sizes of beam-limitapertures and the focusing power of condenser lens 210. Condenser lens210 may be a movable condenser lens that may be configured so that theposition of its first principle plane is movable. The movable condenserlens may be configured to be magnetic, which may result in off-axisbeamlets 212 and 213 illuminating source conversion unit 220 withrotation angles. The rotation angles change with the focusing power orthe position of the first principal plane of the movable condenser lens.Condenser lens 210 may be an anti-rotation condenser lens that may beconfigured to keep the rotation angles unchanged while the focusingpower of condenser lens 210 is changed. In some embodiments, condenserlens 210 may be a movable anti-rotation condenser lens, in which therotation angles do not change when its focusing power and the positionof its first principal plane are varied.

Objective lens 231 may be configured to focus beamlets 211, 212, and 213onto a sample 208 for inspection and may form, in the currentembodiments, three probe spots 221, 222, and 223 on the surface ofsample 208. Gun aperture plate 271, in operation, is configured to blockoff peripheral electrons of primary electron beam 202 to reduce Coulombeffect. The Coulomb effect may enlarge the size of each of probe spots221, 222, and 223 of primary beamlets 211, 212, 213, and thereforedeteriorate inspection resolution.

Beam separator 233 may, for example, be a Wien filter comprising anelectrostatic deflector generating an electrostatic dipole field and amagnetic dipole field (not shown in FIG. 2). In operation, beamseparator 233 may be configured to exert an electrostatic force byelectrostatic dipole field on individual electrons of primary beamlets211, 212, and 213. The electrostatic force is equal in magnitude butopposite in direction to the magnetic force exerted by magnetic dipolefield of beam separator 233 on the individual electrons. Primarybeamlets 211, 212, and 213 may therefore pass at least substantiallystraight through beam separator 233 with at least substantially zerodeflection angles.

Deflection scanning unit 232, in operation, is configured to deflectprimary beamlets 211, 212, and 213 to scan probe spots 221, 222, and 223across individual scanning areas in a section of the surface of sample208. In response to incidence of primary beamlets 211, 212, and 213 orprobe spots 221, 222, and 223 on sample 208, electrons emerge fromsample 208 and generate three secondary electron beams 261, 262, and263. Each of secondary electron beams 261, 262, and 263 typicallycomprise secondary electrons (having electron energy ≤50 eV) andbackscattered electrons (having electron energy between 50 eV and thelanding energy of primary beamlets 211, 212, and 213). Beam separator233 is configured to deflect secondary electron beams 261, 262, and 263towards secondary projection system 250. Secondary projection system 250subsequently focuses secondary electron beams 261, 262, and 263 ontodetection elements 241, 242, and 243 of electron detection device 240.Detection elements 241, 242, and 243 are arranged to detectcorresponding secondary electron beams 261, 262, and 263 and generatecorresponding signals which are sent to controller 50 or a signalprocessing system (not shown), e.g. to construct images of thecorresponding scanned areas of sample 208.

In some embodiments, detection elements 241, 242, and 243 detectcorresponding secondary electron beams 261, 262, and 263, respectively,and generate corresponding intensity signal outputs (not shown) to animage processing system (e.g., controller 50). In some embodiments, eachdetection element 241, 242, and 243 may comprise one or more pixels. Theintensity signal output of a detection element may be a sum of signalsgenerated by all the pixels within the detection element.

In some embodiments, controller 50 may comprise image processing systemthat includes an image acquirer (not shown), a storage (not shown). Theimage acquirer may comprise one or more processors. For example, theimage acquirer may comprise a computer, server, mainframe host,terminals, personal computer, any kind of mobile computing devices, andthe like, or a combination thereof. The image acquirer may becommunicatively coupled to electron detection device 240 of apparatus 40through a medium such as an electrical conductor, optical fiber cable,portable storage media, IR, Bluetooth, internet, wireless network,wireless radio, among others, or a combination thereof. In someembodiments, the image acquirer may receive a signal from electrondetection device 240 and may construct an image. The image acquirer maythus acquire images of sample 208. The image acquirer may also performvarious post-processing functions, such as generating contours,superimposing indicators on an acquired image, and the like. The imageacquirer may be configured to perform adjustments of brightness andcontrast, etc. of acquired images. In some embodiments, the storage maybe a storage medium such as a hard disk, flash drive, cloud storage,random access memory (RAM), other types of computer readable memory, andthe like. The storage may be coupled with the image acquirer and may beused for saving scanned raw image data as original images, andpost-processed images.

In some embodiments, the image acquirer may acquire one or more imagesof a sample based on an imaging signal received from electron detectiondevice 240. An imaging signal may correspond to a scanning operation forconducting charged particle imaging. An acquired image may be a singleimage comprising a plurality of imaging areas. The single image may bestored in the storage. The single image may be an original image thatmay be divided into a plurality of regions. Each of the regions maycomprise one imaging area containing a feature of sample 208. Theacquired images may comprise multiple images of a single imaging area ofsample 208 sampled multiple times over a time sequence. The multipleimages may be stored in the storage. In some embodiments, controller 50may be configured to perform image processing steps with the multipleimages of the same location of sample 208.

In some embodiments, controller 50 may include measurement circuitries(e.g., analog-to-digital converters) to obtain a distribution of thedetected secondary electrons. The electron distribution data collectedduring a detection time window, in combination with corresponding scanpath data of each of primary beamlets 211, 212, and 213 incident on thewafer surface, can be used to reconstruct images of the wafer structuresunder inspection. The reconstructed images can be used to reveal variousfeatures of the internal or external structures of sample 208, andthereby can be used to reveal any defects that may exist in the wafer.

In some embodiments, controller 50 may control motorized stage 209 tomove sample 208 during inspection of sample 208. In some embodiments,controller 50 may enable motorized stage 209 to move sample 208 in adirection continuously at a constant speed. In other embodiments,controller 50 may enable motorized stage 209 to change the speed of themovement of sample 208 overtime depending on the steps of scanningprocess.

Although FIG. 2 shows that apparatus 40 uses three primary electronbeams, it is appreciated that apparatus 40 may use two or more number ofprimary electron beams. The present disclosure does not limit the numberof primary electron beams used in apparatus 40.

Reference is now made to FIG. 3A, which is a schematic diagram ofexemplary multi-beam apparatus illustrating an exemplary configurationof source conversion unit of the exemplary charged particle beaminspection system of FIG. 1, consistent with embodiments of the presentdisclosure. In some embodiments, apparatus 300 may comprise an electionsource 301, a pre-beamlet-forming aperture array 372, a condenser lens310 (similar to condenser lens 210 of FIG. 2), a source conversion unit320 (similar to source conversion unit 120 of FIG. 2), an objective lens331 (similar to objective lens 231 of FIG. 2), and a sample 308 (similarto sample 208 of FIG. 2). Election source 301, pre-beamlet-formingaperture array 372, condenser lens 310, source conversion unit 320, andobjective lens 331 are aligned with a primary optical axis 304 of theapparatus. Electron source 301 generates a primary-electron beam 302along primary optical axis 304 and with a source crossover (virtual orreal) 301S. Pre-beamlet-forming aperture array 372 cuts the peripheralelectrons of primary electron beam 302 to reduce the Coulomb Effect.Primary-electron beam 302 may be trimmed into three beamlets 311, 312and 313 by pre-beamlet-forming aperture array 372 of apre-beamlet-forming mechanism.

In some embodiments, source conversion unit 320 may include abeamlet-limit aperture array 321 with beam-limit apertures configured tolimit beamlets 311, 312, and 313 of primary electron beam 302. Sourceconversion unit 320 may also include an image-forming element array 322with image-forming micro-deflectors, 322_1, 322_2, and 322_3, which areconfigured to deflect beamlets 311, 312, and 313 towards optical axis304 to form virtual images of source crossover 301S. The virtual imagesare projected onto sample 308 by objective lens 331 and form probespots, 391, 392, and 393 thereon. Source conversion unit 320 may furthercomprise an aberration compensator array 324 configured to compensateaberrations of probe spots, 391, 392, and 393. In some embodiments,aberration compensator array 324 may include a field curvaturecompensator array (not shown) with micro-lenses which are configured tocompensate field curvature aberrations of probe spots, 391, 392, and393, respectively. In some embodiments, aberration compensator array 324may include an astigmatism compensator array (not shown) withmicro-stigmators which are configured to compensate astigmatismaberrations of probe spots, 391, 392, and 393, respectively.

In some embodiments, source conversion unit 320 may further comprise apre-bending micro-deflector array 323 with pre-bending micro-deflectors323_1, 323_2, and 323_3 to bend beamlets 311, 312, and 313 respectivelyto be normally incident onto beamlet-limit aperture array 321. In someembodiments, condenser lens 310 may focus three beamlets 311, 313, and313 to become a parallel beam along primary optical axis 304 andperpendicularly incident onto source conversion unit 320.

In some embodiments, image-forming element array 322, aberrationcompensator array 324, and pre-bending micro-deflector array 323 maycomprise multiple layers of micro-deflectors, micro-lenses, ormicro-stigmators.

In source conversion unit 320, beamlets 311, 312 and 313 of primaryelectron beam 302 are respectively deflected by micro-deflectors 322_1,322_2 and 322_3 of image-forming element array 322 towards the primaryoptical axis 304. It is appreciated that beamlet 311 may already be onoptical axis 304 prior to reaching micro-deflector 322_1; accordingly,beamlet 311 may not be deflected by micro-deflector 322_1.

Objective lens 331 focuses beamlets onto the surface of sample 308,i.e., projecting the three virtual images onto the sample surface. Thethree images formed by three beamlets 311-313 on the sample surface formthree probe spots 391, 392 and 393 thereon. The deflection angles ofbeamlets 311-313 are adjusted to reduce the off-axis aberrations ofthree probe spots 391-393 due to objective lens 331, and the threedeflected beamlets consequently pass through or approach the front focalpoint of objective lens 331.

A deflection angle of a beamlet deflected by a micro-deflector (e.g., amicro-deflector in image-forming element array 322) corresponds with aradial shift of the beamlet (i.e., distance from optical axis 304 to thecorresponding beamlet). The deflection angle increases as the radialshift increases. Beamlets having the same radial shifts have the same orsubstantially the same deflection angles. For example, as shown in anexemplary multipole structure array with a 3×3 array configuration inFIG. 3B, the deflection angle of micro-deflector 322_2 may be equal tothe deflection angle of micro-deflector 322_3, if their radial shifts328 and 329 are the same. Moreover, the deflection directions ofbeamlets are related to their corresponding radial shift directions.Furthermore, the aberrations of beamlet (e.g., field curvatureaberrations and astigmatism aberrations) increases as the radial shiftincreases. The aberrations of beamlet having the same or substantiallysame radial shifts are same or substantially same, and the directions ofastigmatism aberrations are related to the directions of their radialshifts.

FIG. 3B shows a 3×3 image-forming micro-deflector array configurationthat can deflect total nine beamlets simultaneously. As the number ofbeamlets increases, the size of the array increases as well. In a largeimage-forming micro-deflector array, therefore, some of beamlets wouldbe located further away from the optical axis (e.g., optical axis 304 ofFIG. 3A) of the apparatus, and the deflection angles thereof increaseaccordingly. Deflection aberrations generated by a micro-deflectorincrease with deflection angle thereof. Therefore, non-uniformity of thecorresponding probe spots increases with the number of beamlets. WhileFIG. 3B shows image-forming micro-deflector array as an example, it isappreciated that a similar relationship between the deflectionaberrations and the size of array may exist in other types ofmicro-deflector arrays.

Reference is now made to FIG. 4, which is an illustration ofdistributions of radial and tangential electrostatic fields of firstorder electric field E_(l) within a deflector. The radial and tangentialcomponents E_(r) and E_(θ) of electric field E at a spatial point (r, θ)inside a deflector with a center axis may be represented as equations(1) and (2), respectively:

$\begin{matrix}{{{E_{r}\left( {r,\theta} \right)} = {\sum\limits_{{k = 1},3,{5\ldots}}^{\infty}{E_{k} \cdot {\cos\left( {k\;\theta} \right)}}}};} & (1) \\{{E_{\theta}\left( {r,\theta} \right)} = {- {\sum\limits_{{k = 1},3,{5\ldots}}^{\infty}{E_{k} \cdot {{\sin\left( {k\;\theta} \right)}.}}}}} & (2)\end{matrix}$

The strength and direction angle α_(k) of the K_(th) order electricfield E_(k) are E_(k)=k·r^(k-1)·d_(k), and α_(k)=kθ. d_(k) is the K_(th)order component on the center axis or called as Kth order on-axiscomponent. Accordingly, E_(l) shown in FIG. 4 is, E_(l)=d₁, and α_(k)=θ.The strength and direction of the first order electric field E_(l) doesnot change with r or θ. Therefore, the first order electric field (i.e.,E_(l) field or d_(l) component) is desired to deflect an electron beam,and the other higher order on-axis components (e.g., d_(2n+1)) need tobe reduced or even eliminated. The larger the deflection angle is, thestronger the E_(l) field or d_(l) component will be needed.

A multi-equal-pole deflector may be defined as a multipole structuredeflector with a center axis and even number of pole electrodes (e.g.,2, 4, 6, 8, 10, so on). In a section normal to the center axis of thedeflector, inner outlines of all pole electrodes are in a circle havinga radius R and equally segmented with segment angle β. For example,FIGS. 5A, 5B, 5C, 5D, and 5E show deflectors with four, six, eight, ten,and twelve pole electrodes, and 90°, 60°, 45°, 36°, and 30° segmentangles, β respectively.

Numbers P, division angles β, and potentials of pole electrodes em in amulti-equal-pole deflector may be configured to generate d1 while makingd2n+1 as small as possible. The pole electrodes em are counted from theX-axis anticlockwise. For example, in FIG. 5A, electrodes 511, 512, 513and 514 are e₁, e₂, e₃ and e₄ respectively. Table 1 shows an excitationsetting of relative excitation voltages that can be applied to eachelectrode e_(n), to generate E₁ parallel to the X axis. In accordancewith Table 1, Table 2 shows the on-axis components of electric fieldswith respect to the number of pole electrodes. By rotating the settingof relative excitation voltages in Table 1 through one or more poleelectrodes, as shown in Table 3, E₁ will accordingly rotate one or moretimes of segment angle β. In Table 3, the excitation setting in Table 1is rotated one time of segment angle for 4-pole deflector and 6-poledeflector, two times of segment angle for 8-pole deflector and 10-poledeflector, three times of segment angle for 12-pole deflector. Combiningthe excitation settings in Table 1 and Table 3 and adjusting basevoltage V1 for Table 1 and base voltage V2 for Table 3, as shown inTable 4, E₁ in any direction and with any strength can be generated, andon-axis components of electric fields with respect to the number of poleelectrodes are the same as Table 2. Furthermore, as shown in Table 2,some of the higher order components of the electric fields may beeliminated by using the deflectors with higher numbers of poleelectrodes.

TABLE 1 e1 e2 e3 e4 e5 e6 e7 e8 e9 e10 e11 e12 4- 1 0 −1 0 pole 6- 1 0.5−0.5 −1 −0.5 0.5 pole 8- 1 0.41 — −1 −1 — 0.41 1 pole 42 0.41 0.41 42 4242 10- 1 0.80 0.30 — — −1 — — 0.30 0.80 pole 90 90 0.30 0.80 0.80 0.3090 90 90 90 90 90 12- 1 0.73 0.26 — — −1 −1 — — 0.26 0.73 1 pole 21 790.26 0.73 0.73 0.26 79 21 79 21 21 79

TABLE 2 d1 d3 d5 d7 d9 d11 d13 d15 d17 *(πR) *(3R³) *(5R⁴) *(7R⁶) *(9R⁸)*(11R¹⁰) *(13R¹²) *(15R¹⁴) *(17R¹⁶) 4-pole −2.8284 d1 −d1 −d1 d1 d1 −d1−d1 d1 6-pole −3 0 d1 −d1 0 −d1 d1 0 d1 8-pole −3.3137 0 0 −d1 d1 0 0−d1 d1 10-pole −3.0902 0 0 0 d1 −d1 0 0 0 12-pole −3.2154 0 0 0 0 −d1 d10 0

TABLE 3 e1 e2 e3 e4 e5 e6 e7 e8 e9 e10 e11 e12 4- 0 1 0 −1 pole 6- 0.5 10.5 −0.5 −1 −0.5 pole 8- 0.41 1 1 0.41 — −1 −1 — pole 42 42 0.41 0.41 4242 10- 0.30 0.80 1 0.80 0.30 — −1 — — pole 90 90 90 90 0.30 0.80 0.800.30 90 90 90 90 12- 0.26 0.73 1 1 0.73 0.26 — — −1 −1 — — pole 79 21 2179 0.26 0.73 0.73 0.26 79 21 21 79

TABLE 4 4-pole 6-pole 8-pole 10-pole 12-pole e1 0.4142 * 0.3090 *(0.2679 * e2 (0 * V1) + (0.5 * V₁) + (0.4142 * (0.8090 * (0.7321 * (1*V₂) (l *) (0.8090 * (0.7321 * e3 (−1 * V₁) + (−0.5 * V₁) + (−0.4142 *V₁) + (0.3090 * V₁) + (0.2679 * V₁) + (0 * V₂) (0.5 * V₂) (1 * V₂) (1 *V₂) (1 * V₂) e4 (0 * V1) + (−1 * V₁) + (−1 * V₁) + (−0.3090 * V₁) +(−0.2679 * V₁) + (−1 * V₂) (−0.5* V₂) (0.4142 * V₂) (0.8090 * V₂) (1 *V₂) e5 (−0.5 * V₁) + (−1 * V₁) + (−0.8090 * V₁) + (−0.7321 * V₁) + (−1 *V₂) (−0.4142 * V₂) (0.3090 * V₂) (0.7321 * V₂) e6 (0.5 * V₁) +(−0.4142 * V₁) + (−1 * V₁) + (−1 * V₁) + (−0.5 * V₂) (−1 * V₂)(−0.3090 * V₂) (0.2679 * V₂) e7 (0.4142 * V₁) + (−0.8090 * V₁) + (−1 *V₁) + (−1 * V₂) (−0.8090 * V₂) (−0.2679 * V₂) e8 (1 * V₁) + (−0.3090 *V₁) + (−0.7321 * V₁) + (−0.4142 * V₂) (−1 * V₂) (−0.7321 * V₂) e9(0.3090 * V₁) + (−0.2679 * V₁) + (−0.8090 * V₂) (−1 * V₂) e10 (0.8090 *V₁) + (0.2679 * V₁) + (−0.3090 * V₂) (−1 * V₂) e11 (0.7321 * V₁) +(−0.7321 * V₂) e12 (1 * V₁) + (−0.2679 * V₂)

For example, as shown in Table 2, a 4-equal-pole deflector has all thehigher order components d_(2n+1). In contrast, a 6-equal-pole deflectordoes not have some of the higher order components (e.g., d₃, d₉, andd₁₅). With a 12-equal-pole deflector, many higher order componentsdisappear (e.g., d₃, d₅, d₇, d₉, d₁₅, and d₁₇). In general, the higherorder components become zero in a period dependent on the pole number ofthe deflector. For example, order k of zero component depends on polenumber (P=4+2p) as shown in equation (3) below:

K=1+2i+n(4+2p)  (3);

where p, n, and i are integers, i=1, 2, . . . p, and n=0, 1, 2, . . . ∞.In some embodiments, the micro-deflectors further away from the opticalaxis of the apparatus (e.g., optical axis 304 of FIG. 3A) may beconfigured to have a higher number of pole electrodes than themicro-deflectors close to the optical axis to reduce more high orderon-axis components.

For the non-zero component d_(k), the corresponding electric fieldE_(k)(r, θ) changes with k−1 power number of the ratio of radialposition r and inner radius R of the deflector as well as the firstcomponent d1. Therefore, Ek can be reduced by reducing the ratio. Insome embodiments, the micro-deflectors further away from the opticalaxis of the apparatus (e.g., optical axis 304 of FIG. 3A) may beconfigured to have a larger inner radius than the micro-deflectors closeto the axis to reduce the non-zero electric fields Ek.

Reference is now made to FIGS. 6A, 6B, and 6C, which are schematicdiagrams of exemplary multipole structure array 622, consistent withembodiments of the present disclosure. Multipole structure array 622 maybe the part of a source conversion unit (such as source conversion unit320 of FIG. 3A). In particular, multipole structure array 622 may be animage-forming element array (such as image-forming element array 322 ofFIG. 3A) or a pre-bending micro-deflector array (such as pre-bendingmicro-deflector array 323 of FIG. 3A). In some embodiments, multipolestructure array 622 may comprise a plurality of micro-deflectors622_1-622_49. With this 7×7 array configuration, forty-nine chargedparticle beams (e.g., electron beams) may be simultaneously deflected toform probe spots on the sample surface. In some embodiments, the centeraxis of the micro-deflector that is located in the middle of the array(e.g., micro-deflector 622_1) may be aligned with optical axis 604 ofthe inspection apparatus (such as optical axis 304 of FIG. 3A). WhileFIGS. 6A, 6B, and 6C show an embodiment of multipole structure arraywith a 7×7 configuration, it is appreciated that the array may be anysize.

Some of forty-nine micro-deflectors 622_1-622_49 are located on theouter portion of the array structure and are further away from opticalaxis 604 than others, thereby having larger radial shifts. For example,the micro-deflectors at the four corners (i.e., micro-deflectors 622_29,622_35, 622_41, and 622_47) are located the farthest away from opticalaxis 604 and may have to generate the largest deflection angles.Moreover, when moving right along the X axis from micro-deflector 622_1to micro-deflector 622_26, the corresponding radial shifts (distancefrom optical axis 604) increase.

To reduce the higher order components of electric fields generated bythese outer micro-deflectors, and thus to reduce the resultingdeflection aberrations and non-uniformity of the corresponding probespots, micro-deflectors with a higher number of pole electrodes may beused to deflect the corresponding beams. In addition, the inner radii Rof micro-deflectors with larger radial shifts may be larger than thosewith smaller radial shifts. Accordingly, moving along the X axis asshown in the FIG. 6A, micro-deflector 622_1 may comprise six poles,micro-deflector 622_2 may comprise eight poles, micro-deflector 622_10may comprise ten poles, and micro-deflector 622_26 may comprise twelvepoles. In some embodiments, the inner radius R of micro-deflector 622_2may be larger than micro-deflector 622_1, the inner radius R ofmicro-deflector 622_10 may be larger than micro-deflector 622_2, and theinner radius R of micro-deflector 622_26 may be larger thanmicro-deflector 622_10.

FIG. 6B illustrates an exemplary grouping of micro-deflectors based onproximity of radial shift from the optical axis. For example, themicro-deflectors having radial shift differences within a <50% range maybe classified into one group. In some embodiments, all micro-deflectorsof a group may use the same type of micro-deflectors. For example, group1 (annotated as G1 in FIG. 6B) comprises the micro-deflector in themiddle of the array (e.g., micro-deflector 622_1), which may comprise a6-pole micro-deflector. Group 2 (annotated as G2 in FIG. 6B) compriseseight micro-deflectors surrounding group 1 (e.g., micro-deflectors622_2-622_9), which may comprise 8-pole micro-deflectors. Group 3(annotated as G3 in FIG. 6B) comprises sixteen micro-deflectorssurrounding group 2 (e.g., micro-deflectors 622_10-622_25), which mayuse 10-pole micro-deflectors. Group 4 (annotated as G4 in FIG. 6B)comprises twenty-four micro-deflectors surrounding group 4 (e.g.,micro-deflectors 622_26-622_49), which may comprise 12-polemicro-deflectors.

Even though, in FIG. 6B, different groups comprise micro-deflectors withdifferent number of poles, it is appreciated that some differing groupsmay comprise micro-deflectors with the same number of poles. Forexample, in some embodiments, micro-deflectors in group 1 and group 2(e.g., micro-deflectors 622_1-622_9) may all use 6-polemicro-deflectors, if the differences of deflection aberrations arewithin an acceptable range between group 1 and group 2. In suchembodiment, group 3 and group 4 may use micro-deflectors with highernumber of poles, such as 8 poles, 10 poles, 12 poles, or higher.

As described earlier with respect to FIG. 6A, for those micro-deflectorswith the same radial shifts from optical axis 604 of the apparatus, suchas the micro-deflectors at the four corners (i.e., micro-deflectors622_29, 622-35, 622_41, and 622_47), the deflection angles for thecorresponding beams may be equal or substantially equal. Furthermore,those micro-deflectors with the same deflection angles may be configuredto have the same orientation angles. Accordingly, in multipole structurearray 622, a group of multi-pole deflectors having same or substantiallysame radial shifts and same or substantially same orientation angles maybe grouped to share a common driver (which performs various controlfunctions, e.g., generating excitation voltages for each electrode,controlling deflection characteristics, and driving the control signalsto micro-deflectors). By sharing a common driver for a plurality ofmicro-deflectors that are configured to deflect the corresponding beamswith the same deflection angles, the number of connecting circuits inthe array configuration may be reduced because a common set of voltagescan be routed to all micro-deflectors in the group. Examples of thedriver sharing technique can be found in U.S. Application No.62/665,451, which is incorporated by reference in its entirety.

FIG. 6C illustrates an exemplary subgrouping of a group ofmicro-deflectors based on the desired deflection angles. Among themicro-deflectors of group 4, the micro-deflectors having same orsubstantially same radial shift may be sub-grouped together and share acommon driver. For example, four micro-deflectors annotated as G4_SG1(the ones at the corners) may be grouped together as sub-group 1.Similarly, eight micro-deflectors annotated as G4_SG2 (neighbors ofsub-group 1) may be grouped together as sub-group 2; eightmicro-deflectors annotated as G4_SG3 (neighbors of sub-group 2) may begrouped together as sub-group 3; and four micro-deflectors annotated asG4_SG4 (the ones on the X axis or the Y axis) may be grouped together assub-group 4. Because the micro-deflectors in each sub-group have thesame or substantially the same radial shift (and thus having the samedeflection angles) and have the same number of poles, thosemicro-deflectors in the sub-group may be connected to a common driver.

Reference is now made to FIG. 7A, which is a schematic diagram of anexemplary multipole structure array 722 with multiple layers, consistentwith embodiments of the present disclosure. Multipole structure array722 may be the part of a source conversion unit (such as sourceconversion unit 320 of FIG. 3A). In particular, multipole structurearray 722 may function as an image-forming element array (such asimage-forming element array 322 of FIG. 3A) or a pre-bendingmicro-deflector array (such as pre-bending micro-deflector array 323 ofFIG. 3A).

In some embodiments, multipole structure array 722 may comprise aplurality of layers of multipole structures, such as layer 722 a and 722b, and each layer may comprise a plurality of multipole structures(e.g., micro-deflectors). For example, layer 722 a may comprisemicro-deflectors 722 a_1-722 a_5. Similarly, layer 722 b may comprisemicro-deflectors 722 b_1-722 b_5. In some embodiments, the center of thelayers may be aligned with an optical axis 704 of the apparatus. Thecenter of the micro-deflector in the middle of each layer (e.g., 722 a_1and 722 b_1) may be aligned with optical axis 704.

In some embodiments, a pair of micro-deflectors, one from each layer,may be aligned together and deflect a corresponding beam. For example,both 722 a_1 and 722 b_1 may deflect beam 711. Similarly, both 722 a_2and 722 b_2 may deflect beam 712; both 722 a_3 and 722 b_3 may deflectbeam 713; both 722 a_4 and 722 b_4 may deflect beam 714; and both 722a_5 and 722 b_5 may deflect beam 715. In a multi-layer configuration,because a pair of micro-deflectors deflect a single beam in series, thedesired deflection angle for each micro-deflector may be smaller than ina single-layer configuration.

In some embodiments, the pair of micro-deflectors may use the same typeof micro-deflector having the same number of poles. For example,micro-deflectors 722 a_1 and 722 b_1 may both comprise 8-polemicro-deflectors. In other embodiments, the pair of micro-deflectors mayuse different types of micro-deflectors. For example, micro-deflector722 a_4 may use a 12-pole micro-deflector, while micro-deflector 722 b_4may use a 10-pole micro-deflector.

Reference is now made to FIG. 7B, which is a schematic diagram of anexemplary layer of multipole structure array of FIG. 7A, consistent withembodiments of the present disclosure. As the number of beams increases,the size of the multipole structure array increases as well. In a largearray of micro-deflectors, therefore, some of beams would be locatedfurther away from the optical axis (e.g., optical axis 304 of FIG. 3A)of the apparatus. The deflection angles of micro-deflectors located atthe outer edge of the array also increase accordingly. Because of thelarge deflection angles, the beams deflected by these micro-deflectorslocated at the outer edge may suffer with higher deflection aberrations,thereby increasing the size and non-uniformity of the correspondingprobe spots. As described with respect to FIG. 6A, in order to reducethe higher order components of electric fields generated by thesemicro-deflectors (thereby reducing the resulting deflection aberrationand non-uniformity of the corresponding probe spots), micro-deflectorswith a higher number of pole electrodes may be used to deflect thecorresponding beams. The same approach may be utilized for a multi-layermicro-deflector array as well.

FIG. 7B shows an example of a 3×3 micro-deflector array, consistent withembodiments of the present disclosure. The array can be a layer of amulti-layer micro-deflector array, such as layer 722 a or 722 b of array722 of FIG. 7A. Nine micro-deflectors 750_1-750_9 may be grouped basedon radial shift, as described with respect to FIGS. 6A and 6B. Forexample, a first group may comprise micro-deflector 750_1, which has thelowest radial shift among nine micro-deflectors. A second group mayinclude the micro-deflectors on the X and Y axis (e.g., micro-deflectors750_2, 750_4, 750_6, 750_8), which have higher radial shifts than thefirst group (e.g., micro-deflector 750_1). A third group may include themicro-deflectors at the four corners (e.g., micro-deflectors 750_3,750_5, 750_7, 750_9), which have the largest radial shifts from opticalaxis 704. Accordingly, the first group (micro-deflector 751_1), thesecond group (micro-deflectors 750_2, 750_4, 750_6, 750_8), and thethird group (micro-deflectors 750_3, 750_5, 750_7, 750_9) may comprise6-pole micro-deflector, 8-pole micro-deflectors, and 12-polemicro-deflectors, respectively, to reduce higher order components ofelectric fields, thereby reducing the resulting deflection aberrationsand non-uniformity of the corresponding probe spots. While FIG. 7B showsa 3×3 array configuration, it is appreciated that the array may be anysize. Furthermore, while FIG. 7B shows three groups each havingdifferent types of micro-deflectors, it is appreciated that the arraymay comprise any combination of groups and types of micro-deflectors.

Furthermore, the driver sharing technique described with respect to FIG.6C may also apply to multi-layer micro-deflector array. For example, thefirst group (micro-deflector 750_1) may be connected and driven by afirst driver. Similarly, all micro-deflectors in the second group(micro-deflectors 750_2, 750_4, 750_6, 750_8) may be connected anddriven by a second driver, because those micro-deflectors have the samedeflection angles. Similarly, all micro-deflectors in the third group(micro-deflectors 750_3, 750_5, 750_7, 750_9) may be connected anddriven by a third driver.

Reference is now made to FIGS. 8A and 8B, which are schematic diagramsof an exemplary multipole structure array with multiple layers,consistent with embodiments of the present disclosure. Multipolestructure array 822 may be the part of a source conversion unit (such assource conversion unit 320 of FIG. 3A). In particular, multipolestructure array 822 may function as an image-forming element array (suchas image-forming element array 322 of FIG. 3A) or a pre-bendingmicro-deflector array (such as pre-bending micro-deflector array 323 ofFIG. 3A).

In some embodiments with a plurality of micro-deflectors, some of theparticle beams may be deflected by micro-deflectors in one layer, whilethe other particle beams may be deflected by micro-deflectors in anotherlayer. For example, beams 811, 814, and 815 may be deflected bymicro-deflector 822 a_1, 822 a_4, and 822 a_5 of layer 822 a, whilebeams 812 and 813 may be deflected by micro-deflectors 822 b_2 and 822b_3 of layer 822 b. By placing some of the micro-deflectors in one layerand the other micro-deflectors in another layer, circuits connecting thepoles in each layer may be reduced in comparison with packing the fullset of micro-deflectors into one layer. This, therefore, may improveelectrical safety and also reduce complexity of design and manufacturingprocess of the multipole structure array.

In some embodiments, layers 822 a and 822 b may include beam path holes822 a_2, 822 a_3, 822 b_1, 822 b_4, and 822 b_5, which let beams passthrough without deflection. As shown in FIG. 8B, because beam path holesare narrower than micro-deflectors (e.g., the width of beam path hole822 a_3 is narrower than the width of micro-deflector 822 a_1), theoverall width of array 822 may be reduced by placing micro-deflectors inalternating fashion as shown in FIGS. 8A and 8B.

Reference is now made to FIGS. 8C, 8D, and 8E, which illustrateschematic diagrams of exemplary layers that can be used within multipolestructure array 822 of FIG. 8A, consistent with embodiments of thepresent disclosure. As the number of beams increases, the size of themultipole structure array increases as well. In a large array ofmicro-deflectors, therefore, some beams are located further away fromthe optical axis (e.g., optical axis 304 of FIG. 3A) of the apparatus.The deflection angles of micro-deflectors located at the outer edge ofstructure array 822 also increases accordingly. Because of the largedeflection angles, the beams deflected by these micro-deflectors locatedat the outer edge may suffer with higher deflection aberrations, therebyincreasing the size and non-uniformity of the corresponding probe spots.As described with respect to FIG. 6A, in order to reduce higher ordercomponents of electric fields generated by these micro-deflectors,thereby reducing the resulting deflection aberrations and non-uniformityof the corresponding probe spots, micro-deflectors with a higher numberof pole electrodes may be used to deflect the corresponding beams. Thesame approach may be utilized for multi-layer micro-deflector array,like array 822 of FIG. 8A, as well.

FIG. 8C shows an exemplary pair of micro-deflector array layers that cansimultaneously deflect 25 particle beams (5×5 configuration). In thisembodiment, micro-deflectors (such as micro-deflector 822 a_1, 822 a_4,822 a_5 of FIG. 8B) and beam path holes (such as beam path holes 822 a_2and 822 a_3 of FIG. 8B) are arranged alternatively in each layer, insuch a way that one particle beam is deflected by a micro-deflector inonly one layer. For example, layer 822 a comprises micro-deflectors 822a_1, 822 a_3, 822 a_5, 822 a_7, 822 a_9, 822 a_10, 822 a_12, 822 a_14,822 a_16, 822 a_18, 822 a_20, 822 a_22, 822 a_24, and beam path holes822 a_2, 822 a_4, 822 a_6, 822 a_8, 822 a_11, 822 a_13, 822 a_15, 822a_17, 822 a_19, 822 a_21, 822 a_23, 822 a_25. Similarly, layer 822 bcomprises micro-deflectors 822 b_2, 822 b_4, 822 b_6, 822 b_8, 822 b_11,822 b_13, 822 b_15, 822 b_17, 822 b_19, 822 b_21, 822 b_23, 822 b_25,and beam path holes 822 b_1, 822 b_3, 822 b_5, 822 b_7, 822 b_9, 822b_10, 822 b_12, 822 b_14, 822 b_16, 822 b_18, 822 b_20, 822 b_22, 822b_24. Accordingly, 13 of 25 beams are deflected by micro-deflectors oflayer 822 a, while the remaining 12 beams are deflected bymicro-deflectors of layer 822 h.

Like previous embodiments, micro-deflectors with the same or similarradial shifts (e.g., radial shift differences within a <50% range may begrouped together and have a certain number of pole electrodes to reducethe deflection aberrations. For example, in layer 822 a, micro-deflector822 a_1 is a 6-pole micro-deflector, micro-deflectors 822 a_3, 822 a_5,822 a_7, and 822 a_9 are 8-pole micro-deflectors, and micro-deflectors822 a_10, 822 a_12, 822 a_14, 822 a_16, 822 a_18, 822 a_20, 822 a_22,and 822 a_24 are 10-pole micro-deflectors. Similarly, in layer 822 h,micro-deflectors 822 b_2, 822 b_4, 822 b_6, and 822 b_8 are 8-polemicro-deflectors, and micro-deflectors 822 b_11, 822 b_13, 822 b_15, 822b_17, 822 b_19, 822 b_21, 822 b_23, and 822 b_25 are 10-polemicro-deflectors. While FIG. 8C shows a 5×5 array configuration, it isappreciated that the array may be any size. Also, it is appreciated thatthe array configuration may comprise any combination of groups and typesof micro-deflectors.

Furthermore, the driver sharing technique described with respect toFIGS. 6C and 7B may also apply to this embodiment. In layer 822 b, forexample, all micro-deflectors in a first group (micro-deflector 822 b_2,822 b_4, 822 b_6, 822 b_8) may be connected and driven by a firstdriver, because those micro-deflectors have the same deflection angleand the same numbers of pole electrodes. Similarly, all micro-deflectorsin a second group (micro-deflectors 822 b_11, 822 b_13, 822 b_15, 822b_17, 822 b_19, 822 b_21, 822 b_23, and 822 b_25) may be connected anddriven by a second driver.

FIGS. 8D and 8E show another embodiment of micro-deflector array layerthat can simultaneously deflect forty-nine particle beams (7×7configuration). In this embodiment, micro-deflectors with a certainnumber of pole electrodes are placed in one layer. For example, allmicro-deflector with six poles or ten poles are placed in layer 822 a ofFIG. 8D, while all micro-deflectors with eight poles and twelve polesare placed in layer 822 b of FIG. 8E. Among the total of forty-ninebeams, seventeen beams are deflected by layer 822 a, and the remainingthirty-two beams are deflected by layer 822 b. In layer 822 a, a firstgroup includes micro-deflector 822 a_1 in the middle, which may comprisesix poles. A second group includes micro-deflectors 822 a_10-822 a_25,which may comprise ten poles. Similarly, in layer 822 b, a third groupincludes micro-deflectors 822 b_2-822 b_9, which may comprise eightpoles. A fourth group includes micro-deflectors 822 b-26-822 b_49, whichmay comprise twelve poles. Furthermore, the driver sharing techniquedescribed with respect to FIGS. 6C and 7B may also apply to thisembodiment. For example, in the layer 822 b of FIG. 8E, the fourmicro-deflectors in the third group that are positioned on one of the X-or Y-axis (e.g., micro-deflector 822 b_2, 822 b_4, 822 b_6, 822 b_8) maybe connected and driven by one common driver, because thosemicro-deflectors have the same deflection angles, same orientationangles, and the same numbers of pole electrodes. Similarly, the otherfour micro-deflectors in the third group that are positioned on thecorners (e.g., micro-deflector 822 b_3, 822 b_5, 822 b_7, 822 b_9) maybe connected and driven by another common driver.

Reference is now made to FIG. 9, which is a flow chart illustrating anexemplary method of manufacturing an exemplary configuration of amultipole structure array, consistent with embodiments of the presentdisclosure. In some embodiments, the multipole structure array may bemanufactured using a semiconductor fabrication process. In someembodiments, the multipole structure array may comprise amicro-deflector array, such as micro-deflector array 622 of FIG. 6A. Insome embodiments, the multipole structure array may comprise a pluralityof micro-deflectors, such as micro-deflectors 622_1-622_49 of FIG. 6A.To reduce the higher order components of electric fields generated bythe micro-deflectors, thereby reducing the resulting deflectionaberrations and non-uniformity of the corresponding probe spots,micro-deflectors with higher number of pole electrodes may be used todeflect the corresponding beams that are further away from an opticalaxis of an inspection apparatus. For example, a first multipolestructure (such as micro-deflector 622_29 of FIG. 6A) having a higherradial shift (i.e., further away from the optical axis) than a secondmultipole structure (such as micro-deflector 622_3 of FIG. 6A), maycomprise a micro-deflector with a higher number of pole electrodes thanthe second micro-deflector.

In step 910, the number of pole electrodes of the first multipolestructure is configured based on deflection aberration characteristic ofthe first multipole structure. In step 920, the number of poleelectrodes of the second multipole structure is configured based ondeflection aberration characteristic of the second multipole structure.The number of pole electrodes selected for the second multipolestructure in step 920 is less than the number of pole electrodesselected for the first multipole structure in step 910.

In step 930, the first multipole structure is formed at a location witha first radial shift from a central axis of the array. In step 940, thesecond multipole structure is formed at a location with a second radialshift from a central axis of the array. The distance between the opticalaxis and the location of the first multipole structure is greater thanthe distance between the optical axis and the location of the secondmultipole structure. Accordingly, the first multipole structure has alarger radial shift than the second multipole structure.

It is appreciated that the first and second multipole structures can bepart of separate groups of multipole structures as explained above withrespect to; for example, FIG. 6B. Moreover, it is appreciated that thefirst and second multipole structures can be located on separate layersas explained above with respect to, for example, FIG. 8C.

The embodiments may further be described using the following clauses:

1. A micro-structure deflector array including a plurality of multipolestructures, each multipole structure comprising a plurality of poleelectrodes, the array comprising:

a first multipole structure of the plurality of multipole structures,the first multipole structure having a first radial shift from a centralaxis of the array; and

a second multipole structure of the plurality of multipole structures,the second multipole structure having a second radial shift from thecentral axis of the array,

wherein the first radial shift is larger than the second radial shift,and the first multipole structure comprises a greater number of poleelectrodes than the second multipole structure.

2. The array of clause 1, wherein the first multipole structurecomprises a greater number of pole electrodes than the second multipolestructure to reduce deflection aberrations when the plurality ofmultipole structures deflects a plurality of charged particle beams.3. The array of any one of clauses 1 and 2, wherein:

the plurality of pole electrodes of the first multipole structure areelectrically connected and driven by a first driver, and the pluralityof pole electrodes of the second multipole structure are electricallyconnected and driven by a second driver.

4. The array of clause 3, wherein the first driver and the second driverare configured to enable the first multipole structure and the secondmultipole structure to function as image-forming elements or pre-bendingmicro-deflectors in a multi-beam apparatus to deflect the plurality ofcharged particle beams.5. The array of any one of clauses 1-4, wherein the first multipolestructure has an inner diameter larger than the second multipolestructure.6. A micro-structure deflector array including a plurality of multipolestructures, each multipole structure comprising a plurality of poleelectrodes, the array comprising:

a first group of multipole structures of the plurality of multipolestructures, the first group of multipole structures having a first setof radial shifts from a central axis of the array, wherein eachmultipole structure of the first group comprises a same number ofcorresponding pole electrodes; and

a second group of multipole structures of the plurality of multipolestructures, the second group of multipole structures having a second setof radial shifts from the central axis of the array, wherein eachmultipole structure of the second group comprises a same number ofcorresponding pole electrodes,

wherein the lowest value of radial shift of the first set of radialshifts are higher than the highest value of radial shift of the secondset of radial shifts, and a multipole structure of the first groupcomprises a greater number of pole electrodes than a multipole structureof the second group.

7. The array of clause 6, wherein the multipole structure of the firstgroup comprises a greater number of pole electrodes than the multipolestructure of the second group to reduce deflection aberrations when theplurality of multipole structures deflects a plurality of chargedparticle beams.8. The array of any one of clauses 6 and 7, wherein the first group orthe second group may only comprise one multipole structure.9. The array of any one of clauses 6-8, wherein the first group ofmultipole structures of the plurality of multipole structures comprises:

a first sub-group of multipole structures that are electricallyconnected and driven by a first driver, wherein the radial shifts andorientation angles of the first sub-group of multipole structures areequal or substantially equal.

10. The array of any one of clauses 6-9, wherein the second group ofmultipole structures of the plurality of multipole structures comprises:

a second sub-group of multipole structures that are electricallyconnected and driven by a second driver, wherein the radial shifts andorientation angles of the second sub-group of multipole structures areequal or substantially equal.

11. The array of any one of clauses 6-10, wherein at least one of thefirst and second drivers is configured to enable the correspondingmultipole structures to function as image-forming elements orpre-bending micro-deflectors in a multi-beam apparatus to deflect theplurality of charged particle beams.12. The array any one of clauses 6-11, wherein one multipole structureof the first group has an inner diameter larger than one multipolestructure of the second group.13. A micro-structure deflector array including a plurality of multipolestructures configured to deflect a plurality of charged particle beams,each multipole structure comprising a plurality of pole electrodes, thearray comprising:

a first layer of multipole structures of the plurality of multipolestructures, the first layer comprising a first multipole structurehaving a first radial shift from a central axis of the array and asecond multipole structure having a second radial shift from the centralaxis of the array, wherein the first radial shift is larger than thesecond radial shift, and the first multipole structure comprises agreater number of pole electrodes than the second multipole structure;and

a second layer of multipole structures of the plurality of multipolestructures, the second layer comprising a third multipole structurehaving a third radial shift from the central axis of the array.

14. The array of clause 13, wherein the first multipole structurecomprises a greater number of pole electrodes than the second multipolestructure to reduce deflection aberrations of the corresponding chargeparticle beams.15. The array of any one of clauses 13 and 14, wherein the third radialshift is smaller than the first radial shift.16. The array of clause 15, wherein the third radial shift is largerthan the second radial shift.17. The array of any one of clauses 13-16, wherein the number of poleelectrodes of the third multipole structure is larger than or equal tothe second multipole structure.18. The array of any one of clauses 13-16, wherein the number of poleelectrodes of the third multipole structure is smaller than or equal tothe first multipole structure.19. The array of any one of clauses 13, 14, 16, and 17, wherein thethird multipole structure comprises a greater or equal number of poleelectrodes than the first multipole structure.20. The array of any one of clauses 13-19, wherein one of the pluralityof charged particle beams is deflected by a multipole structure of thefirst layer, and another one of the plurality of charged particle beamsis deflected by a multipole structure of the second layer.21. The array of any one of clauses 13-19, wherein one of the pluralityof charged particle beams is deflected by a multipole structure of thefirst layer and a multipole structure of the second layer in series, andthe multipole structure of the first layer and the multipole structureof the second layer are aligned each other.22. The array of any one of clauses 13-19, wherein:

a first beam of the plurality of charged particle beams is deflected bya multipole structure of the first layer,

a second beam of the plurality of charged particle beams is deflected bya multipole structure of the second layer, and

a third beam of the plurality of charged particle beams is deflected bya multipole structure of the first layer and a multipole structure ofthe second layer in series.

23. The array of any one of clauses 13-22, wherein each multipolestructures of the plurality of multipole structures is placed inside anelectrically shielding cavity to be electrically shielded from othermultipole structures.24. The array of any one of clauses 13-23, wherein the first multipolestructure of the first layer has an inner diameter larger than thesecond multipole structure of the first layer.25. The array of any one of clauses 13-24, wherein two or more ofmultipole structures of the first layers:

have a same number of pole electrodes,

are equal or substantially equal in radial shift and orientation angle,and

are electrically connected and driven by a first driver.

26. The array of any one of clauses 13-25, wherein two or more ofmultipole structures of the second layers:

have a same number of pole electrodes,

are equal or substantially equal in radial shift and orientation angle,and

are electrically connected and driven by a second driver.

27. A source conversion unit in a charged particle beam systemcomprising the array of any one of clauses 13-26.28. A method of manufacturing a micro-structure deflector arrayincluding a plurality of multipole structures, each multipole structurecomprising a plurality of pole electrodes, the method comprising:

forming the first multipole structure to have a first radial shift froma central axis of the array; and

forming the second multipole structure to have a second radial shiftfrom the central axis of the array, wherein the first radial shift islarger than the second radial shift and the first multipole structurehas a different number of pole electrodes from the second multipolestructure.

29. The method of clause 28, further comprising selecting the number ofpole electrodes of the first multipole structure and the number of poleelectrodes of the second multipole structure based on aberrationcharacteristics of the first and second multipole structures.30. The method of clause 29, wherein selecting the number of poleelectrodes of the first multipole structure and the number of poleelectrodes of the second multipole structure comprises selectingcorresponding numbers of pole electrodes to reduce high-order componentsof electric fields thereof.31. The method of any one of clauses 28-30, wherein the number of poleelectrodes of the first multipole structure is larger than the secondmultipole structure.32. The method of any one of clause 28-31, further comprising placingthe plurality of multipole structures in one or more layers.33. The method of clause 32, wherein one multipole structure in a firstlayer of the one or more layers is aligned with one multipole structurein a second layer of the one or more layers.34. The method of any one of clauses 28-33, further comprising groupinga subset of multipole structures to share one driver, wherein the subsetof multipole structures:

have a same number of pole electrodes, and

are equal or substantially equal in radial shift and orientation angle.

35. The method of any one of clauses 32-34, further comprising groupinga subset of multipole structures in the first layer of the one or morelayers to share a first driver, wherein the subset of multipolestructures in the first layer:

have a same number of pole electrodes, and

are equal or substantially equal in radial shift and orientation angle.

36. The method of any one of clauses 32-35, further comprising groupinga subset of multipole structures in the second layer of the one or morelayers to share a second driver, wherein the subset of multipolestructures in the second layer: have a same number of pole electrodes,and are equal or substantially equal in radial shift and orientationangle.37. A micro-structure deflector array including a plurality of multipolestructures, each multipole structure comprising a plurality of poleelectrodes, the array comprising:

a first group of multipole structures of the plurality of multipolestructures, the first group of multipole structures having a first setof radial shifts from a central axis of the array, wherein eachmultipole structure of the first group comprises a same number ofcorresponding pole electrodes; and

a second group of multipole structures of the plurality of multipolestructures, the second group of multipole structures having a second setof radial shifts from the central axis of the array, wherein eachmultipole structure of the second group comprises a same number ofcorresponding pole electrodes,

wherein a multipole structure of the first group comprises a greaternumber of pole electrodes than a multipole structure of the secondgroup, and wherein each of the first group and the second groupcomprises one or more multipole structures.

38. The array of clause 37, wherein a lowest value of radial shift ofthe first set of radial shifts are higher than a highest value of radialshift of the second set of radial shifts.39. The array of clause 37, wherein the plurality of multipolestructures are configured to substantially simultaneously deflect aplurality of charged particle beams.40. The array of clause 39, wherein the first group of multipolestructures comprises:

a first sub-group of multipole structures that are electricallyconnected to, and driven by, a first driver, wherein radial shifts andorientation angles of the first sub-group of multipole structures aresubstantially equal.

41. The array of clause 40, wherein the second group of multipolestructures comprises:

-   -   a second sub-group of multipole structures that are electrically        connected to, and driven by, a second driver, wherein radial        shifts and orientation angles of the second sub-group of        multipole structures are equal or substantially equal.        42. The array of clause 41, wherein one of the first and second        drivers is configured to enable corresponding multipole        structures to deflect the plurality of charged particle beams in        a multi-beam apparatus, and wherein further the plurality of        multipole structures are configured as image-forming elements or        pre-bending micro-deflectors in the multi-beam apparatus.        43. The array of clause 37, wherein a multipole structure of the        first group has an inner diameter larger than a multipole        structure of the second group.        44. The array of clause 39, wherein the first group and the        second group are arranged in a first layer of the array, and        wherein the array further comprises a second layer that        comprises a third group of multipole structures having a third        radial shift from the central axis of the array.        45. The array of clause 44, wherein the third radial shift is        different than the first set of radial shifts or the second set        of radial shifts.        46. The array of clause 45, wherein a number of pole electrodes        of the third multipole structure is different than, or equal to,        a number of pole electrodes of a multipole structure of the        first group of multipole structures.        47. The array of clause 45, wherein one of the plurality of        charged particle beams is deflected by a multipole structure of        the first layer, and another one of the plurality of charged        particle beams is deflected by a multipole structure of the        second layer.        48. The array of clause 45, wherein one of the plurality of        charged particle beams is deflected by a multipole structure of        the first layer and a multipole structure of the second layer in        series, and the multipole structure of the first layer and the        multipole structure of the second layer are aligned to each        other.        49. The array of clause 45, wherein:

a first beam of the plurality of charged particle beams is deflected bya multipole structure of the first layer and is not deflected by anymultipole structure of the second layer,

a second beam of the plurality of charged particle beams is deflected bya multipole structure of the second layer and is not deflected by anymultipole structure of the second layer and

a third beam of the plurality of charged particle beams is deflected bya multipole structure of the first layer and a multipole structure ofthe second layer in series.

50. A source conversion unit in a charged particle beam system, whereinthe source conversion unit comprises a micro-structure deflector arrayincluding a plurality of multipole structures, each of the multipolestructures comprising a plurality of pole electrodes, the arraycomprising:

a first group of multipole structures of the plurality of multipolestructures, the first group of multipole structures having a first setof radial shifts from a central axis of the array, wherein eachmultipole structure of the first group comprises a same number of poleelectrodes; and

a second group of multipole structures of the plurality of multipolestructures, the second group of multipole structures having a second setof radial shifts from the central axis of the array, wherein eachmultipole structure of the second group comprises a same number of poleelectrodes,

wherein the first set of radial shifts is different from the second setof radial shifts, wherein a multipole structure of the first groupcomprises a greater number of pole electrodes than a multipole structureof the second group, and wherein each of the first group and the secondgroup comprises one or more multipole structures.

51. The source conversion unit of clause 50, wherein the first group andthe second group are arranged in a first layer of the array, the arrayfurther comprising a second layer that comprises a third group ofmultipole structures having a third radial shift from the central axisof the array.52. The array of any one of clauses 6-10, wherein a driver of the firstand second drivers is configured to enable the corresponding multipolestructures to function as image-forming elements or pre-bendingmicro-deflectors in a multi-beam apparatus to deflect the plurality ofcharged particle beams.53. The array of clause 52, wherein the driver of the first and seconddrivers being configured includes all of the first and second driversbeing configured.

While the present invention has been described in connection withvarious embodiments, other embodiments of the invention will be apparentto those skilled in the art from consideration of the specification andpractice of the invention disclosed herein. It is intended that thespecification and examples be considered as exemplary only, with a truescope and spirit of the invention being indicated by the followingclaims.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made as described without departing from the scope of the claimsset out below.

1-15. (canceled)
 16. A micro-structure deflector array including aplurality of multipole structures, each multipole structure comprising aplurality of pole electrodes, the array comprising: a first multipolestructure of the plurality of multipole structures, the first multipolestructure having a first radial shift from a central axis of the array;and a second multipole structure of the plurality of multipolestructures, the second multipole structure having a second radial shiftfrom the central axis of the array, wherein the first radial shift islarger than the second radial shift, and the first multipole structurecomprises a greater number of pole electrodes than the second multipolestructure.
 17. The micro-structure deflector array of claim 16, whereinthe first multipole structure comprises a greater number of poleelectrodes than the second multipole structure to reduce deflectionaberrations when the plurality of multipole structures deflects aplurality of charged particle beams.
 18. The micro-structure deflectorarray of claim 17, wherein: the plurality of pole electrodes of thefirst multipole structure are electrically connected and driven by afirst driver, and the plurality of pole electrodes of the secondmultipole structure are electrically connected and driven by a seconddriver.
 19. The micro-structure deflector array of claim 18, wherein thefirst driver and the second driver are configured to enable the firstmultipole structure and the second multipole structure to function asimage-forming elements or pre-bending micro-deflectors in a multi-beamapparatus to deflect the plurality of charged particle beams.
 20. Themicro-structure deflector array of claim 16, wherein the first multipolestructure has an inner diameter larger than an inner diameter of thesecond multipole structure.
 21. The micro-structure deflector array ofclaim 16, wherein: the plurality of pole electrodes of the firstmultipole structure are electrically connected and driven by a firstdriver, and the plurality of pole electrodes of the second multipolestructure are electrically connected and driven by a second driver. 22.A method of manufacturing a micro-structure deflector array including aplurality of multipole structures, each multipole structure comprising aplurality of pole electrodes, the method comprising: forming a firstmultipole structure to have a first radial shift from a central axis ofthe array; and forming a second multipole structure to have a secondradial shift from the central axis of the array, wherein the firstradial shift is larger than the second radial shift, and wherein thefirst multipole structure has a different number of pole electrodes fromthe second multipole structure.
 23. The method of claim 22, furthercomprising selecting a number of pole electrodes of the first multipolestructure and a number of pole electrodes of the second multipolestructure based on aberration characteristics of the first and thesecond multipole structures.
 24. The method of claim 23, whereinselecting the number of pole electrodes of the first multipole structureand the number of pole electrodes of the second multipole structurecomprises selecting corresponding numbers of pole electrodes to reducehigh-order components of electric fields thereof.
 25. The method ofclaim 22, wherein the number of pole electrodes of the first multipolestructure is greater than the number of pole electrodes of the secondmultipole structure.
 26. The method of claim 22, further comprisingplacing the plurality of multipole structures in one or more layers. 27.The method of claim 26, wherein one multipole structure in a first layerof the one or more layers is aligned with one multipole structure in asecond layer of the one or more layers.
 28. The method of claim 22,further comprising grouping a subset of multipole structures to shareone driver, wherein the subset of multipole structures: have a samenumber of pole electrodes, and are equal or substantially equal inradial shift and an orientation angle.
 29. The method of claim 27,further comprising grouping a subset of multipole structures in thefirst layer of the one or more layers to share a first driver, whereinthe subset of multipole structures in the first layer: have a samenumber of pole electrodes, and are equal or substantially equal inradial shift and an orientation angle.
 30. The method of claim 27,further comprising grouping a subset of multipole structures in thesecond layer of the one or more layers to share a second driver, whereinthe subset of multipole structures in the second layer: have a samenumber of pole electrodes, and are equal or substantially equal inradial shift and an orientation angle.
 31. A non-transitory computerreadable medium that stores a set of instructions that is executable byat least one processor of a computer system to cause the computer systemto perform a method of manufacturing a micro-structure deflector arrayincluding a plurality of multipole structures, each multipole structurecomprising a plurality of pole electrodes, the method comprising:forming a first multipole structure to have a first radial shift from acentral axis of the array; and forming a second multipole structure tohave a second radial shift from the central axis of the array, whereinthe first radial shift is larger than the second radial shift, andwherein the first multipole structure has a different number of poleelectrodes from the second multipole structure.
 32. The non-transitorycomputer readable medium of claim 31, wherein the set of instructions isexecutable by the at least one processor to cause the computer system tofurther perform selecting a number of pole electrodes of the firstmultipole structure and a number of pole electrodes of the secondmultipole structure based on aberration characteristics of the first andthe second multipole structures.
 33. The non-transitory computerreadable medium of claim 32, wherein selecting the number of poleelectrodes of the first multipole structure and the number of poleelectrodes of the second multipole structure comprises selectingcorresponding numbers of pole electrodes to reduce high-order componentsof electric fields thereof.
 34. The non-transitory computer readablemedium of claim 31, wherein the number of pole electrodes of the firstmultipole structure is greater than the number of pole electrodes of thesecond multipole structure.
 35. The non-transitory computer readablemedium of claim 31, wherein the set of instructions is executable by theat least one processor to cause the computer system to further performplacing the plurality of multipole structures in one or more layers. 36.The non-transitory computer readable medium of claim 35, wherein onemultipole structure in a first layer of the one or more layers isaligned with one multipole structure in a second layer of the one ormore layers.
 37. The non-transitory computer readable medium of claim31, wherein the set of instructions is executable by the at least oneprocessor to cause the computer system to further perform grouping asubset of multipole structures to share one driver, wherein the subsetof multipole structures: have a same number of pole electrodes, and areequal or substantially equal in radial shift and an orientation angle.38. The non-transitory computer readable medium of claim 36, wherein theset of instructions is executable by the at least one processor to causethe computer system to further perform grouping a subset of multipolestructures in the first layer of the one or more layers to share a firstdriver, wherein the subset of multipole structures in the first layer:have a same number of pole electrodes, and are equal or substantiallyequal in radial shift and an orientation angle.
 39. The non-transitorycomputer readable medium of claim 36, wherein the set of instructions isexecutable by the at least one processor to cause the computer system tofurther perform grouping a subset of multipole structures in the secondlayer of the one or more layers to share a second driver, wherein thesubset of multipole structures in the second layer: have a same numberof pole electrodes, and are equal or substantially equal in radial shiftand an orientation angle.